gh-38053: work on letterplace algebras
Moving the letterplace implementation of free algebras to the `Parent`
framework
### :memo: Checklist
- [x] The title is concise and informative.
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URL: https://github.com/sagemath/sage/pull/38053
Reported by: Frédéric Chapoton
Reviewer(s): Frédéric Chapoton, Matthias Köppe, Travis Scrimshaw
